An 8 bit Adder/Subtractor Unit

An 8 bit Adder/Subtractor Unit
Two 4 bit 74LS283 chips can be cascaded together to form an 8 bit parallel adder unit. Each of the two 74LS283 IC(s) is connected to the 1^st complement circuitry that allows either the un-complemented form for subtraction to be applied at the inputs of the two 74LS283 IC(s).
Two set of AND – OR based circuit that allows complemented and non complemented B input to be applied at the B input of the two 4 bit adders. The add/ Subtract function select input are tied together. The carry-In of the 1^st 4 bit adder circuit is connected to the add / subtract function select input. The carry-out of the 1^st 4 adder circuit is connected to the carry-In of the 2^nd 4 bit adder circuit.

Figure: 8 bit Adder/ Subtractor circuit

Consider two numbers A = 103 and B = 67 which are first added and then subtracted using the 8 bit Adder/ Subtractor circuit.

Table: Adding 103 and 67

Table: Subtracting 103 and 67